look at the instructions inside the forearm. The magnum can be set up for lighter shells. My A5 magnums liked regular 2 3/4 High brass fine, the longer forcing cone has a nice effect. True if it is a full choke that steel shot bigger than #4 should not be used. They did make magnums with looser barrels and yours can be loosened up reasonably as the magnum barrels are heavier and have the meat to ream, you can even have Colonial steel certified sleeves installed which is how I went. True the magnums aren't as valuable as the lt weight ones are.